This invention relates to a process for electrochemical reduction of terephthalic acid, as the sodium salt, to the sodium salt of p-hydroxymethylbenzoic acid, and to improvements in product isolation and purification, wherein the catholyte comprises a soluble sodium compound, and the pH of the catholyte is controlled within limits to obtain good current efficiency and little or no by-products, such as ring-reduced compounds, coupled materials and amines.
Electrochemical reduction of the ammonium salt of terephthalic acid yields the ammonium salt of p-hydroxymethylbenzoic acid (p-HMBA). However, the ammonium salt of p-HMBA is difficult to purify because of the difficulty of crystallizing the ammonium salt without the inclusion of impurities in the ammonium crystals. The electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid in ammonia solution is taught in commonly-assigned applications Ser. No. 319,120 and Ser. No. 358,222, incorporated herein by reference, and in German Pat. No. 2,642,496.
Electrochemical reduction of the sodium salt of terephthalic acid yields the sodium salt of p-HMBA under controlled conditions. The sodium salt of p-HMBA, in the presence of water, forms a crystal with five molecules of water which crystallizes readily in a pure crystalline form which is easily separated from other impurities. However, usual electrochemical reduction of the sodium salt of terephthalic acid without control of pH results in ring reduction of the terephthalic acid with consequent yield losses.
This invention relates to p-hydroxymethylbenzoic acid. More particularly, it relates to a process for the preparation of p-hydroxymethylbenzoic acid with a low level of impurities. Even more particularly, the invention relates to a process for preparation of p-hydroxymethylbenzoic acid (p-HMBA) by electrochemical reduction of sodium terephthalate to produce the sodium salt of p-HMBA, wherein ring reduction of the terephthalic acid and production of by-products are minimal.
Numerous methods are known for the preparation of p-hydroxymethylbenzoic acid (p-HMBA). Some of these methods are based on the saponification of a corresponding halogenmethyl compound, such as p-chloromethylbenzoic acid, or the esters thereof, or p-chloromethylbenzonitrile. For example, several methods for the synthesis of p-hydroxymethylbenzoic acid are taught in U.S. Pat. No. 4,130,179, incorporated herein by reference.
p-Hydroxymethylbenzoic acid must be free from by-products when it is to be employed in polycondensation reactions, such as in the preparation of polyesters. However, most of the known processes for the preparation of p-hydroxymethylbenzoic acid do not yield the acid free from by-products. Thus, for example, during the saponification of highly pure p-chloromethylbenzoic acid in a faintly alkaline aqueous medium, up to 10% of dibenzylether-4,4'-dicarboxylic acid is always produced.
It is well-known that in the cathodic reduction of carboxylic acids that two types of products can result, either the corresponding aldehyde in a two-electron process or the hydroxymethyl compound in a four-electron process where the aldehyde is further reduced to the alcohol. (M. Baizer, Organic Electrochemistry, Dekker, N.Y. (1973), p. 414) The alcohol can be further reduced to the methyl group. Reduction of the benzene ring of aromatic dicarboxylic acids has also been observed to yield other impurities. (P. C. Condit, I&EC, 48(8), 1252(1956); U.S. Pat. Nos. 2,477,579; 2,477,580; 3,471,381; and 3,542,656.)
Accordingly, in the preparation of p-hydroxymethylbenzoic acid by whatever method used to obtain the crude acid, many by-products can also be produced, among which are 4-carboxybenzaldehyde, dihydroxymethylbenzene, toluic acid, coupled products and ring-reduced products. Hydrogenation of terephthalic acid in an electrochemical process results in quantities of 4-carboxybenzaldehyde despite the concurrent hydrogenation of 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid. 4-Carboxybenzaldehyde is a particularly undesirable impurity because it acts as a chain-stopper during polyesterification. 4-carboxybenzaldehyde is difficult to remove by physical means but it can be hydrogenated to the hydroxymethyl compound, i.e., p-hydroxymethylbenzoic acid, in an electrochemical process as is taught by Baizer, mentioned above. Toluic acid acts as a chain-stopper during polyesterification but toluic acid can be efficiently removed by cooling and crystallizing crude p-hydroxymethylbenzoic acids containing it. Ring-reduced by-products reduce current efficiency as measured in product yield.
Accordingly, under the usual conditions used to obtain reduction of terephthalic acid to p-hydroxymethylbenzoic acid, the presence of the resulting by-products in the product stream renders the resulting p-hydroxymethylbenzoic acid unfit for polyesterification without further extensive purification. Current efficiency also can be low. Methods of purification of p-hydroxymethylbenzoic acid are detailed in commonly assigned application Ser. No. 445,659, incorporated herein by reference.
In a typical purification of crude p-hydroxymethylbenzoic acid prepared electrochemically in aqueous ammonium solution, the first step is preferably hydrogenation of the 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid (p-HMBA). Any suitable catalyst, such as platinum or palladium, can be used. Noble metal catalysts, such as platinum on carbon, are preferred. Typical hydrogenation processes are taught in U.S. Pat. No. 3,726,915; German Offen. No. 2,045,747; Japanese Kokai Tokyo Koho No. 80,143,933; Belgium Pat. No. 876,860; German Offen. No. 2,709,525 and U.S. Pat. No. 4,260,817.
Another procedure is to distill off the water content of the hydrogenated ammonium salt of p-HMBA, then vacuum decompose the residue to drive off the ammonia of the salts of p-HMBA at temperatures of about 115.degree. C. and a pressure of below 3 mm Hg. An alternative procedure is to vacuum steam decompose the ammonium salts of p-HMBA at about 200.degree. C. and 0.2 mm Hg to obtain the free acid, p-HMBA, which contains impurities.
Treatment of the free acid by hot aqueous filtration removes terephthalic acid. The free acid obtained by decomposing the ammonium salt is dissolved in water and filtered at a temperature within the range of from about 80.degree. C. to about 130.degree. C., preferably from about 110.degree. C. to about 120.degree. C. under pressure of from about 15 to 50 psi. The terephthalic acid, being less soluble in hot water than p-hydroxymethylbenzoic acid, solubilizes to a limited amount and is removed by filtration.
p-Toluic acid contaminants can be removed from the free acid of p-HMBA obtained by the ammonium process by extraction with p-xylene from aqueous solutions. The extraction can be performed at temperatures within the range of from about 23.degree. C. to about 175.degree. C., preferably from about 23.degree. C. to about 150.degree. C.
The pure p-HMBA in aqueous solution after extraction of p-toluic acid is thereupon obtained by cooling and crystallization. The filtrate is recycled.
An alternative procedure to obtain the free acid from the crude ammonium salt of p-HMBA is acidification with a mineral acid. The free acid, p-HMBA, is dissolved in excess hot water. The slurry is filtered to remove terephthalic acid, the free terephthalic acid being less soluble in water of a temperature of 80.degree. C. to 130.degree. C. under pressure of from about 15 to 50 psi, preferably 110.degree. C. to 120.degree. C., than the free acid of p-HMBA. Cooling of the filtrate yields p-HMBA.
Crude p-HMBA from the ammonium process can also be purified by forming the acetate of the acid, followed by vacuum distillation, recrystallization of the acetate and hydrolysis, as is taught in commonly assigned U.S. Pat. No. 4,182,847.
Since the acid monomer, p-hydroxymethylbenzoic acid, can also be obtained by hydrolysis of the ester of the acid, the purified acid can also be obtained by purification of the ester prior to hydrolysis of the acid.
Purification of p-hydroxymethylbenzoic acid in the form of the sodium salt, as is taught in U.S. Pat. No. 3,534,089, incorporated herein by reference, provides an improved method which overcomes the disadvantages of the ammonium process wherein the ammonium salt is the product of the electrochemical reduction of terephthalic acid. However, simple replacement of the ammonium moiety by a sodium moiety after reduction of terephthalic acid in an ammonium solution is not economically feasible because of product losses due to formation of the disodium salt of unreacted terephthalic acid, which must be removed for recycle. As previously noted, extensive ring formation occurs if the sodium salt of p-HMBA is prepared by the electrochemical reduction of sodium terephthalate by previously known methods.
Accordingly, it is an object of the present invention to provide an electrochemical process for the reduction of terephthalic acid wherein the electrochemical product is the sodium salt of p-hydroxymethylbenzoic acid. A further object of this invention is to provide a process for electrochemical manufacture of p-hydroxymethylbenzoic acid from terephthalic acid wherein ring reduction does not occur or is significantly less than that obtained with the disodium salt of terephthalic acid, and the level of impurities after recrystallization is below 100 ppm of terephthalic acid. A still further object of this invention is to provide a process for the electrochemical reduction of terephthalic acid, and subsequent purification wherein the purification can be performed easily and simply using the sodium salt of p-HMBA. These and other objects of the present invention will become apparent to those skilled in the art from a reading of the following.